1. Field of the Invention
The present invention relates to a print processor that utilizes a page printer.
2. Description of the Related Art Along with the recent development of small, high-speed xerographic page printers suitable for digital printing has come the widespread use of novel print processors that go beyond the traditional concept of text-centered information printing. Such print processors utilize description languages that allow images, graphics and texts to be uniformly processed altogether. The processing includes enlargement, rotation, transformation and other desired handling of graphics and texts. PostScript (TM of Adobe Systems, U.S.), Interpress (TM of Xerox Corp., U.S.), Acrobat (TM of Adobe Systems) and GDI (Graphics Device Interface, TM of Microsoft Corp., U.S.) are well-known, representative description languages.
Print data prepared in description language include descriptive instructions arranged in a desired order for describing images, graphics and texts positioned as desired within a page. To have such data printed by a printer related to the present invention requires that the print data be rasterized into raster graphics before printing. The expansion involves forming raster scanning lines by rasterizing the target data into a series of individual dots or pixels arranged across part of or all of a page, the scanning lines being generated successively from top to bottom within one page. Conventional page printers rasterize the print data of the entire page into raster graphics before printing and have the send data stored in a page buffer memory. To store the raster data about the whole page requires installing a memory of a large capacity. State-of-the-art xerographic color page printers require page buffer memories of particularly large capacities for two reasons: because the printers need to deal with toners of four colors (cyan (C), magenta (M), yellow (Y) and black (Bk)), and because the printers generally use a plurality of bits of information per pixel to meet the demand for higher picture quality than monochromatic page printers.
Against the background of the massive memory requirement, the so-called band memory technique has been proposed recently as a viable way to reduce the memory capacity and to lower costs accordingly. The band memory technique does not envisage rasterizing all print data of a single page into raster graphics before printing by a page printer. Instead, the target print data prepared in description language are converted to relatively simple intermediate data that may be rasterized into raster graphics faster than the original print data. The page is divided into a plurality of contiguous regions called bands. The intermediate data representing the respective bands are first stored and then transferred successively to a raster graphics rasterizing unit whereby the intermediate data are placed in buffer memory locations corresponding to the bands. The band memory technique requires a new memory arrangement for storing the intermediate data but achieves reductions in the capacity of the buffer memory for accommodating massive raster data. Generally, however, the band memory technique requires that before the end of the printing of raster data about a given band, the expansion of intermediate data into raster data for the next band be completed. In such cases, the expansion of the intermediate data into raster data may not be finished in time if the print data include instructions for drawing complicated graphics or for drawing images of huge data quantities, or if a specific band within a page comprises instructions for drawing complicated graphics or images.
A number of solutions to the above problems regarding the band memory technique have been proposed. Disclosures that bear on the present invention are made illustratively by Japanese Published Unexamined Patent Application Nos. Hei 6-290007 and Hei 6-344639.
Japanese Published Unexamined Patent Application No. Hei 6-290007 discloses a print processor which measures the time required to rasterize first-format intermediate data into raster data per band. If the expansion is not finished in time for a specific band, the expansion of first-format intermediate data into raster data is followed by lossless compression of the data, the result being stored as second-format intermediate data. However, the presence of second-format intermediate data runs counter to the initial purpose of the band memory technique, i.e., it can lead to a larger memory for storing the intermediate data. Thus whenever a predetermined capacity of the intermediate data memory is exceeded, the second-format intermediate data are again rasterized into raster data. The rasterizing process is followed by lossy compression of the data at a high compression rate. The result is stored as third-format intermediate data. That is, the print processor described in Japanese Published Unexamined Patent Application No. Hei 6-290007 allows a page printer to output all pages even where print data include instructions for drawing complicated graphics or images, with a price paid for a worsened image quality due to the lossy compression at high compression rate. Furthermore, additional processes are needed to implement a plurality of intermediate data formats for the data to be accommodated in suitable memories. This can prolong the processing time of the entire print operation.
Japanese Published Unexamined Patent Application No. Hei 6-344639 discloses a print processor which causes a host computer to transfer input data to a page printer while measuring the transfer speed of the input data. On the basis of the input data transfer speed thus measured, the disclosed print processor controls the print speed of the page printer so as to avert situations where the expansion of the target data into raster data is not finished in time. Specifically, if the input data transfer speed is high, the printing speed of the page printer is increased correspondingly; the lower the input data transfer speed, the lower the printing speed of the page printer. However, the data to be input to the page printer can only be handled in a format in which the number of input data items is proportional to the number of output data items (the disclosed embodiment assumes input data to be composed of font data and raster data alone). Thus the print processor disclosed by Japanese Published Unexamined Patent Application No. Hei 6-344639 is incapable of handling print data containing instructions in description language for drawing complicated graphics.